1. Field of the Invention
This invention relates to the field of asphalt compositions, particularly compositions containing cationic substances which improve the adhesion of the asphalt to mineral aggregate.
2. Description of the Prior Art
In road construction, bitumen-aggregate mixtures are applied to the road surface. These bitumen-aggregate mixtures can be obtained by mixing anionic or cationic asphalt emulsions with aggregate, such as stone chips, gravel or sand, or by mixing free flowing heated asphalt (asphalt cement) with pre-dried aggregate, by a hot mix process. The pre-dried aggregate can also be mixed with asphalt diluted with a hydrocarbon solvent, known as cutback asphalt.
The quality of the road surface is generally dependent upon the strength of the bonds between the asphalt and aggregate after setting of the composition. Poor service performance is due to poor adhesion between the asphalt and aggregate, resulting in the stripping off of the asphalt from the aggregate surface.
Asphalt compositions have relatively poor adhesion to aggregate in the presence of water. Since the aggregate is preferentially wetted by water, even if the aggregate is dry at the time it is blended with the asphalt, the eventual penetration of water into the composition reaches the aggregate and interferes with the bond between the aggregate and the asphalt. The result of this stripping is flaked pavement and pot holes. Stripping problems also generally occur if the aggregate is poorly dried, if sandy carbonate aggregate containing a large amount of quartz particles is used, if carbonate aggregate is covered with dust or if igneous (silicate) aggregates, such as granite, diorite, gabbro, diabase, or basalt, that strip in the presence of external water are used.
To avoid such failures, adhesion improving agents known as "anti-stripping agents" are commonly added to the asphalt. Before the mixing operation, these agents are added to the bituminous binder to reduce its surface tension and to induce on the binder an electrical charge opposite to that of the aggregate surface. Lower surface tension gives improved wettability of the aggregate and charge reversal enhances bond strength by increasing Coulomb's attractive forces.
Cationic substances, particularly amines, have been traditionally used as anti-stripping agents. The cationic substances increase the hydrophobicity of the aggregate, making the aggregate resistant to the penetration of water so that water seeping into the asphalt does not tend to destroy the bond between the asphalt and the aggregate. The addition of the cationic substances tends to make the aggregate sufficiently water resistant that a good bond with the asphalt is formed. Among the cationic materials which have been used as adhesion promoters with asphalt are primary alkyl amines such as lauryl amine and stearyl amine, and the alkylene diamines, such as the fatty alkyl substituted alkylene diamines. Because these amines rapidly lose their activity when combined with asphalt and stored at elevated temperatures for an extended period, it has therefore been necessary to combine the amine with the asphalt at the work site when the asphalt is combined with the aggregate, which in practice presents problems in obtaining a homogeneous mixture. There is also a danger since these amines are corrosive.
The patent literature sets forth a large number of compounds which can be used to improve adhesion of aggregate and asphalt. These include fatty carboxylic chromites (U.S. Pat. No. 3,963,059), combinations of epoxy resins and onium borates (U.S. Pat. No. 3,947,395), and ether amines in combination with alkanolamines (U.S. Pat. No. 3,928,061). U.S. Pat. No. 3,868,263 discloses a method of making a paving composition where the adhesion promoter is a primary, secondary, teritary or quaternary amine or imidazoline, or reaction product of these with various acids including fatty acids, metal soaps, and several other compounds including rosin reaction products. Also known are ethylene oxides condensates of long chain alkyl triamines (U.S. Pat. No. 3,615,797), alkoxylated fatty amines and their salts (U.S. Pat. No. 3,347,690) and reaction products of ozonized unsaturated fatty acids with polyalkylene amines (U.S. Pat. Nos. 3,246,008 and 3,245,451).
One promising class of compounds used as adhesion promoters are aminoamines formed from the reaction of tall oil fatty acid fractions and inexpensive polyamines. Since these low cost polyamines are scarce, the commercial potential of these materials is quite limited.
A fatty aminoester has also become available for use as an adhesion promoter under the name PAVEBOND AP sold by Carstab. This produce has low amine value (AV=56) and low viscosity. Low viscosity is important in adhesion promoting agents, since no fuel oil or other diluent need be added in order to effectively mix the adhesion promoter and the asphalt.